Research On The Key Material For All Vanadium Redox Flow Battery

The all vanadium redox flow battery (VRB) is an energy storage technique suitable for using in large stafionary application. As a kind of large energy storage system, it also has a lot of advantages, such as, adjustable capacity and power, long cycle life, flexible design, high efficiency and high safety. The most important thing is that it uses a single metal ion in both electrolytes and avoids cross-contamination between two half-cell electrolyte. However, a lot of problems still exist from theory to practical application of VRB. From the materials aspect, the development of VRB mainly is restricted by three key materials, that is, electrode, membrane and electrolyte. In order to obtain favorable energy storage performance, the appropriate electrode materials should be selected to improve the electrode activity forwards electrochemical reaction, or modified by deliberate treatment. The membrane material also should be development to improve its selective permeability and extend the cycle life. In parallel, economic and effective approaches should be developed to improve the stability of electrolyte. All of these have important significance to improve the energy density and efficiency of VRB and speed up its commercialization process.At first, this thesis describes the study on the preparation and characterization of V(Ⅲ)/V(Ⅳ) mixed electrolyte. A simple preparation method was selected using equal molar amount of V2O3 and V2O5 as raw material dissolved in 4.5 M sulfuric acid solution. Then VOSO4 solution was obtained by heating the solution Afterwards, a mixture solution of V(Ⅲ) and V(Ⅳ) was obtained by electrolysis, and V3.5+ electrolyte was obtained. We used (NH4)2Fe(SO4)2 titration, UV-Vis spectrophotometer and CV to characterize the prepared electrolyte.This thesis also includes the studies on the modification of graphite felt with N dopant activation method, oxidation activation method and vanadium compound activation method. The results show that gelatin has activity effect with the felt, but ammonia not. The graphite felt treated by oxidation exhibits improved activity with the reaction of V(Ⅳ)/V(Ⅴ) redox couple, while the felt treated by vanadium solution exhibits improved activity with the reaction of V(Ⅱ)/V(Ⅲ) redox couple. Moreover, the activity of the modified graphite felt increases with increasing solution concentration of the vanadium compounds, and the temperature change during the treatment shows no effect to increase the felt activity.Finally the charge and discharge performance of the vanadium redox cell was tested using the prepared electrolyte and modified graphite felts. After 30 charge/discharge cycles, the coulomb efficiency and voltage efficiency of the cell using the pristine felt slightly declines. The coulomb efficiency gradually increases and voltage efficiency decreases with the increasing of current density. And energy efficiency increases before it reduces.That at current density of 28 mA/cm2, the coulomb efficiency, voltage efficiency and energy efficiency were 85.92%,94.66% and 80.93%, respectively. It was the best performance condition of this cell. At last, the influence of modified electrode was used the cell resulting in improved coulomb efficiency, but decreased voltage efficiency and energy efficiency.